1. Field of the Invention
The present invention relates to Faraday rotators, optical isolators, polarizers and diamond-like carbon thin films, and more particularly relates to—in optical communications fields—Faraday rotators for rotating light-wave polarization planes, optical isolators for blocking return beams from a light source, polarizers for transmitting only a given polarized component of light, and to diamond-like carbon thin films utilized as materials in optical communications fields.
2. Background Art
In optical communications systems constituted from optical fibers and optical elements, reflected light from optical-connector junctions and optical circuit components is sometimes reintroduced to the light source. Noise produced by beams returning to a light source—especially to a semiconductor laser—often turns out to be a major problem in designing optical communications systems and optical devices.
The means commonly used for blocking off the return beams is an optical isolator, whose constituent elements are a Faraday rotator, a polarizer, an analyzer, and a magnetic part.
By virtue of the magnetic part applying a magnetic field to a magneto-optical body (magneto-optical material), Faraday rotators rotate the polarization plane of an incident light beam traveling in the direction of the magnetic field. Meanwhile, polarizers (analyzers) allow only a given polarized light component to pass, and block components apart from that which is polarized.
As illustrated in FIG. 14, an optical isolator 6 is configured as an assembly of a polarizer 2, a Faraday rotator 3, an analyzer 4, and a magnetic part 5, and the non-repelling characteristics of the magneto-optical material are exploited to block the incident light from being reintroduced from the opposite direction. A general optical-isolator assembly will be more specifically described in the following, while reference is made to FIG. 14.
Incident light from a light source 1 initially is filtered through the polarizer 2 into a polarization plane, and then transits the Faraday rotator 3, whereby the polarization plane is rotated 45 degrees. With its polarization plane rotated by 45 degrees the incident light passes through and radiates as it is from the analyzer 4, and in part once more enters the analyzer 4 as a return beam and is reintroduced into the Faraday rotator 3. The polarization plane of the return beam is again rotated 45 degrees by the Faraday rotator 3, and with its polarization plane having been rotated 90 degrees in total, the return beam is unable to pass through the polarizer 2, where the return beam is thus blocked off.
It will be understood that the arrows drawn at certain angles with respect to the arrows indicating either the light emitted from the light source 1 or the return beam are schematic representations of the polarization directions of either the emitted light or the return beam.
Yttrium iron garnet (YIG hereinafter) crystals or bismuth-substituted garnet crystals have usually been used for conventional Faraday rotators (magneto-optical bodies). Furthermore, for conventional polarizers (analyzers), rutile (titanium oxide) monocrystals or glass superficially onto which silver particles are orientated in a single direction are usually used, while for the magnetic part that applies a magnetic field to the magneto-optical body, samarium-based rare-earth magnetic substances are used.
The YIG crystals or bismuth-substituted garnet crystals chiefly used for conventional Faraday rotators must have a certain thickness to obtain a needed Faraday rotation angle, which results in a large external form. Likewise, the external form becomes large in the case of the rutile monocrystals and the glass onto which silver particles are superficially orientated in a single direction, that have been chiefly used for conventional polarizers (analyzers), and the samarium-based rare-earth magnetic substances chiefly used as the magnetic part for applying a magnetic field to the magneto-optical body, since they must occupy a certain volume. What is more, with conventional isolators especially—whose basic constituent elements are a Faraday rotator, a polarizer (analyzer) and a magnetic part—has been the problem of being large-sized overall.
Meanwhile, Faraday rotators, polarizers (analyzers) and magnetic bodies are expensive, making conventional optical isolators in which these are the constituent elements cost all the more. A further problem has been that because the individual constituent elements in conventional optical isolators are independent, their assembly process is complex, adding that much more to the cost.
Moreover, because as a general rule what determines a Faraday-rotator angle is its thickness, conventional Faraday rotators can only correspond to a single wavelength. The consequent problem too with conventional optical isolators having a conventional Faraday rotator as a constituent element has been that they basically can handle only a single wavelength.